1. Field of the Invention
The present invention relates to a zoom lens system which includes a wide-angle range.
2. Description of Related Art
An image sensor used in a compact digital camera typically has a size of about 1/2.5-inch through 1/1.7-inch, in which high pixelization is achieved by miniaturizing the pixel pitch. However, in recent years, the pixel pitch has been reduced down to approximately 1 through 2 μm, and further improvement of the image quality cannot be expected. One way to achieve high pixelization is to use a large-sized image sensor in which the pixel pitch can be enlarged even in a compact digital camera. However, if the image sensor is enlarged, the optical system also increases in size to the extent that such an optical system cannot be accommodated in a compact digital camera. In particular, in a focusing method of the related art in which an entire lens group (so as to constitute a focusing lens group) such as a frontmost lens group or rearmost lens group is moved along the optical axis direction, it is difficult to miniaturize a lens unit which includes a focusing mechanism for such a focusing lens group.
A zoom lens system having three lens groups, i.e., a negative lens group, a positive lens group and a negative lens group, in that order from the object side, and a zoom lens system that has favorable telecentricity having a negative lens group, a positive lens group and a positive lens group, in that order from the object side, are known in the art as examples of lens systems which are aimed at achieving a wide angle-of-view and a reduced length in the optical axis direction. However, in the former zoom lens system, focusing is often carried out by the first lens group, and in the latter zoom lens system, focusing is often carried out by the third lens group, in which both of these first and third lens groups have large diameters; furthermore, since the amount of movement required for either of these lens groups is also comparatively large, there is a problem with there being a large mechanical burden.
In an effort to solve this problem, a zoom lens system having three lens groups, i.e., a negative lens group, a positive lens group and a positive lens group, in that order from the object side, is disclosed in Japanese Patent No. 4,067,828, which attempts to achieve miniaturization and a reduction in weight of the zoom lens system by using the lens element provided closest to the image side within the second lens group as a focusing lens group.
However, in the zoom lens system disclosed in the above-mentioned Japanese Patent No. 4,067,828, since the refractive power of the focusing lens group is weak, the amount of movement (along the optical axis) of this focusing lens group during a focusing operation is large, so that the reduction of mechanical burden of the focusing mechanism, and the miniaturization of the zoom lens system are insufficient.